1. Field of the Invention
The invention relates generally to downhole injection of one or more tracers or marker materials in a well.
2. Background Art
When a well, specifically an oil or gas well, has been completed and is yielding a desired product, it is necessary to monitor the well's performance to ensure that it is behaving as expected. In particular, it is desirable to measure a velocity of a fluid along a borehole and up to the surface. In an oil well, the fluid may be oil, water, gas or a combination, even a mixture, of all three. It is generally desirable to monitor the velocities of the fluids actually down the well itself rather than merely when they reach the surface.
Many types of method and apparatus have been proposed for this purpose. A first method involves the use of a mechanical “spinner>>: a wireline-supported tool carrying a small propeller- (or turbine-) driven dynamo is placed in a flowing fluid so that the propeller is turned around by it, and the dynamo's output indicates the velocity of the flowing fluid. The first method provides satisfactory results in borehole sections that are vertical, but not in sections which are horizontal. A horizontal section may indeed comprise several layers, e.g. an oil layer above a water layer.
A second method involves an injector/detector tool that injects a small amount of tracer e.g., a detectable chemical or a radioactive substance, at a first location and detects the injected tracer at a second location. The flow velocity is calculated by a simple distance over time calculation. An example of injector/detector tool is the Tracer Injector tool of Schlumberger which is described in U.S. Pat. Nos. 4,166,215 and 4,166,216. The U.S. Pat. No. 6,125,934 describes an injector tool adapted for a horizontal section. For a determined tracer intended to be ejected into a corresponding layer, the injector tool comprises an ejection port that is oriented such that the determined tracer may be ejected directly into the corresponding layer. Hence an oil miscible tracer is injected into an oil layer and a water miscible tracer is injected into a water layer.
FIG. 1 illustrates a typical injector/detector tool known from prior art. An injector tool 101 ejects a tracer into a borehole 102 at a first location (not shown in FIG. 1), and a detector tool 103 detects the tracer at a second location (not shown in FIG. 1). The ejection of the tracer is commanded via a command wire 106 that controls from a surface system 111 located at the surface a solenoid valve 105 of the injector tool. When the switch is on, the solenoid valve 105 is open and a relatively constant force acting on a piston 107 due to a spring 104 expels an amount of the tracer through a thin tube 112.
FIG. 1 illustrates a system as it is being used in a vertical well. However, such a system may also be used in a horizontal well. For this purpose, it may further comprise an ejection port that is judicially oriented to aim a determined layer.
A dedicated wire 108 transmits an analog signal to activate the detector tool 103, thus allowing to detect the tracer. When activated, the detector tool transmits results to the surface system 111 via a results wire 115. The electrical wires (106, 108, 115) are located within a central chamber 113 so as to be protected from any liquid. The central chamber 113 may also contain one or more additional wires that are used for a communicating between the surface system 111 and one or more additional devices, e.g. a logging device 109.
In the system illustrated in FIG. 1, the tracer is directly ejected from a reservoir 114. The quantity of tracer that is discharged into the borehole 102 is determined by a duration of the opening of the solenoid valve 105, the duration being controlled at surface. However, it is not possible to insure that the quantity of tracer corresponding to the duration has effectively been ejected.
In another system from prior art, the quantity of tracer that is periodically discharged into the borehole space is determined by a size of a syringe filled with the tracer before ejection. The tracer may be stored in a reservoir that communicates with the syringe so as to provide a regular filling of the syringe. A Hall Effect sensor may detect the end of stroke of the piston so as to confirm that the quantity of tracer has been ejected.
When a piece of the injector tool has to be replaced, e.g., the reservoir 114, the injector tool needs to be completely stripped out. It is indeed necessary to cut electrical wires located in the central chamber 113 to remove the piece.
Similarly, the electro-valve 105 comprises a solenoid coil 117 communicating with the command wire 106, and a solenoid seat 118 through which the tracer is expelled. The solenoid coil 117 is used to control a movement of a plunger 119 that allows the tracer to be expelled. When the solenoid seat 118 is replaced, the solenoid coil also needs to be removed.